Several types of storage and retrieval systems are now in use, all of which are designed to provide as much storage capacity as possible on a minimum of floor area, in order to conserve valuable warehouse space. A satisfactory system must also provide means for quickly determining the location of any stored article, for removing it from storage, and for directing incoming stock to the location designated for its storage.
An older type of storage system comprises stationary racks arranged along relatively wide aisles to be accessible to a lift truck which can be driven along the aisles to any desired rack and which has an elevatable fork for retrieving articles from the racks. A generally similar system, but requiring less aisle space, comprises a stacker crane or similar carrier which serves the same purpose as the lift truck but which moves along the aisles on a track and which can swivel to either side of the aisle to be operable in aisles too narrow for a lift truck. One stacker crane may serve an entire warehouse, or there may be a dedicated stacker crane for each aisle. As with a lift truck, a stacker crane may be controlled by an operator who rides on the crane itself, although there is increasing installation of automated apparatus wherein the stacker crane is controlled by a computer.
Some further reduction in floor area needed for a given storage capacity is achieved with a revolving carousel comprising upright bins connected laterally like the links of a chain and guided by floor rails for movement in a long, narrow closed loop whereby a bin containing a desired article can be brought to a loading and unloading station at one end of the loop, where an operator manually removes the article from the bin. The carousel apparatus eliminates the need for a moving vehicle to carry articles between the storage location and the operator's station, and it lends itself to automation and computerized control. It has some disadvantages, however. Whenever a new bin must be brought to the loading and unloading station, the whole chain of bins has to be moved, and the operator cannot be stocking a bin or removing articles from it when the bins are moving. Hence the operator is sometimes compelled to be idle, waiting for a bin to be brought to the loading and unloading station from some distance away. Another significant disadvantage is that the chain of bins cannot turn square corners, and therefore the rows of bins along the opposite long sides of each loop must be spaced apart. Thus, even though adjacent carousels can be placed close together, with no significant unused floor space between them, there is a significant amount of unusable floor area inside the loop of each carousel.
In a sense, a lift truck carries the operator to the load whereas an automated stacker crane and a carousel move the load to the operator. In general, it is more efficient for the operator to remain at a fixed station where he can accomplish paperwork, operate a computer terminal and the like; but some of the potential efficiency of a load-to-operator system is sacrificed if too much time is required for bringing a desired article to the operator's station.
An important consideration in any storage and retrieval system is security against pilferage. The open racks used in a lift truck system offer a minimum of such security. A carousel system is superior in this respect because it can be enclosed in a fence or the like that prevents access to all bins except the one at the operator's station.
Thus satisfactory storage and retrieval apparatus should provide for a maximum of storage capacity in a minimum of floor area, be capable of computer control to bring a desired article to the operator rather than requiring an operator to go to the location where the article is stored, bring the article to the operator as quickly as possible, and provide good security against pilferage. An obvious further requirement is that the apparatus be low in cost.